Nonetheless, a few prospective studies have provided key information on the frequency of PTS after symptomatic DVT. In a longitudinal cohort study of patients with a first episode of acute symptomatic DVT, Prandoni et al1 found that the cumulative incidence of PTS was 17.3% after 1 year (severe in 3%), 23% after 2 years, 28% after 5 years (severe in 9%), and 29% after 8 years. In a subsequent trial to evaluate the use of compression stockings to prevent PTS in patients with symptomatic proximal DVT, mild to moderate PTS occurred in 20% of patients assigned to stockings and in 47% of controls, and severe PTS occurred in 11% and 23%, respectively.2 The frequency of PTS in the stockings group was similar to that in the study by Prandoni et al, in which all patients were encouraged to wear compression stockings. In both studies, most cases of PTS occurred within 2 years of DVT. In contrast, in a recent study by Ginsberg and colleagues,53 27% of patients (none of whom used stockings) had developed PTS by 1 year after a first episode of symptomatic proximal DVT, but among patients who were free of PTS 1 year after DVT, only 5% subsequently developed PTS (average follow-up, 55 months). However, the Ginsberg et al study used stricter diagnostic criteria for PTS: in addition to symptoms, objective demonstration of valvular incompetence was required.